1. Field of the Invention
This invention relates generally to apparatus and method for forming extruded ribbons of self-adhering elastomeric material.
2. Description of the Related Art
Extruded ribbons of self-adhering elastomeric materials recently have come into commercial usage, in applications where the self-adhesive character of such materials is advantageously employed, e.g., in products such as disposable diapers, and in shoe covers where the ribbons are used, wherein these ribbons are utilized as elastic gathers around leg openings, as foot opening gathering means and anti-skid strips on the soles thereof.
The elastomeric materials from which such ribbons are extruded may be self-adherent per se or may be rendered self-adherent by, for example, the addition of tackifiers to the elastomeric material prior to hot-melt formation and extrusion thereof in ribbon form. Illustrative self-adhering elastomeric materials include those described in U.S. Pat. No. 4,418,123 to Bunnelle, et al., incorporated herein by reference. Commercially available materials of such type include FULLASTIC.RTM. adhesive elastic as available from H. B. Fuller Company.
A problem which has arisen in the production of extruded ribbons of such self-adherent elastomeric materials has been the nonuniformity of physical properties arising from the method of cooling the extrudate ribbons. Such cooling typically has been carried out by passing the hot-melt extrudate of the elastomeric material onto the outer cylindrical surface of a rotating chill roll. The chill roll may be internally cooled or alternatively may be mounted in partially submerged position in a water bath, to effect dissipation of heat from the ribbon on the chill roll outer cylindrical surface. The cooling of the extrudate ribbon by means of such chill roll systems yields nonuiform properties, e.g., tack and dynamic adhesion, across the thickness of the ribbon, due to the preferential cooling of one side thereof. The prior art has proposed various approaches to cooling of extrudate and hot-melt materials, but the same are characterizable by deficiencies in application to self-adhering elastomeric ribbon materials.
U.S. Pat. No. 3,175,026 to A. L. James discloses a system for extrusion of thermoplastic film materials such as polyethylene, in substantially fluid condition onto a cooled arcuate surface and then into a nip formed with an adjacent arcuate surface to form sheets of uniform gauge. The sheet thus is cooled to solidified condition while advancing in surface contact over the arcuate surfaces, both of which are smooth and glossy, one being relatively deformable. The arcuate surfaces, both of which may be cooled, advance at a faster rate of speed than the rate of extrusion to pull the sheet of fluid thermoplastic material from the extrusion source.
A specifically disclosed embodiment in the patent comprises a cooled metallic cylinder cooperating with a draw roll and rotated therewith or independently driven at the same peripheral speed in opposite directions. The film of thermoplastic material solidified on the cooling cylinder is withdrawn therefrom by means of guide rolls to a final winding station where the film material is wound into a roll. The patent also discloses an embodiment which utilizes in place of the draw roll an endless backup belt mounted on a series of drive rollers and positioned in contact with the cooling cylinder to support the thermoplastic film about a portion of the periphery thereof (column 5, lines 35-37). The patent discloses the use of a hollow internally cooled drum to constantly cool the surface of the belt to prevent overheating thereof.
The method and apparatus of the James patent utilizes arcuate conveying surfaces of different materials and different sizes (the draw roll or endless belt providing a smaller extent of thermoplastic film-contacting surfaces than the cooling cylinder) so that primary cooling and solidification is effected on the cooling cylinder. The thermoplastic sheet material in the James system thus is subjected to intrinsically dissimilar rates and ranges of cooling on its respective opposite surfaces, consistent with the teaching in the patent at column 4, lines 38-41 that "it has been found desirable to extrude the curtain onto the cooled arcuate metallic surface of the drum 8 and then to the nip in actual practice to quickly cool the film." Accordingly, even if the system were modified to obtain the same heat transfer rates on both surfaces of the film in the nip between the cylinder and draw roll or belt, the thermoplastic material would already have been subjected to a preliminary cooling on one surface prior to concurrent cooling of both surfaces.
U.S. Pat. No. 2,590,186 to E. H. Land discloses a method of forming a solid film on the surface of a sheet-like material from a viscous liquid mass of the film-forming material. Sheet-like materials are fed along conveying paths with viscous liquid being fed to the sheets to provide a body of liquid therebetween. At least one of the sheets is absorbent of the solvent for the film-forming material, so that a substantial amount of drying of the formed film takes place rapidly due to such solvent absorption. When drying has reached a point where the film is a substantially continuous solid, at least one of the sheets is separated from the forming film and continued drying takes place due to evaporation of remaining solvent from the exposed surface of the film. The patent discloses in column 4, lines 55-67 to pass the supporting sheets having the film-forming material therebetween, through an oven at elevated temperature, or alternatively to expose them to dry hot air, infrared lamps, etc., to enhance the removal of solvent from the film-forming composition.
U.S. Pat. No. 3,852,387 to N. M. Bortnick, et al. discloses using paired endless belts for forming extruded polymer melts into thermoplastic sheets. A strand of extruded polymer melt is taken up between the moving belts at a temperature that allows the melt to adhere to the belts while being formed into a web by compression, spreading and flattening therebetween. The endless belts preferably utilize smooth, polished metallic surfaces to produce flat, optical quality thermoplastic sheet.
A plurality of compressed air knives are spaced intermediate the runs of the belts to enhance cooling of the thermoplastic material, if ambient cooling proves inadequate, to a temperature allowing parting of the product sheet without adherence to either belt.